What Causes Strep Throat While On Cephalexin?

Hello, Doctor. I recently took Cephalexin as prescribed for an infected toe. The toe is fine now, but the podiatrist mentioned that some anti-biotics may harm the body s natural immune system by destroying the good bacteria in our system. Yesterday, I was diagnosed with strep and candida for two maladies in my oral, throat and sinus cavities, requiring four additional medicines. The PA who diagnosed me also alluded to the possibility that Cephalexin may have contributed to the current situation. I m 69 years old, generally good health, etc., and don t get sick but maybe once every 2-3 years. Any thoughts?